1. Field of the Invention
The present invention is directed to a method of operating an injection molding apparatus utilizing an acceptable part and process window for controlling the process whereby the dimension, weight and stress in the molded article, as well as the cycle time, may be closely controlled.
2. Description of the Prior Art
The production of consistent and uniform product by the injection molding process has been a long standing objective of the injection molding industry. This objective has become more difficult to achieve as more and more products are produced by this process which are ever increasingly complex and have ever tighter tolerances. The objective has been further complicated by the trend towards fewer and fewer operators associated with the injection molding machines, requiring greater automatic control of the process and apparatus.
A variety of interrelated parameters of the material, machine, and mold must be accomodated by any automated control system for the satisfactory operation of an injection molding apparatus; among these parameters are the type of material being molded, the consistency of the plastic characteristics, the molding cycle time, the machine shot size, melt viscosity and temperature consistency, mold clamp pressure, and injection pressure, among others. It has been found that, as each of these parameters varies during the operation of an injection molding machine, the product uniformity may suffer without constant operator attention.
Many attempts have been made at automatically controlling the injection molding process to produce uniform and consistent product, yet none of the proposed solutions has gained widespread acceptance, at least partially due to the fact that the proposed solutions do not satisfactorily address all of the variables involved.
Among the solutions proposed are those taught by U.S. Pat. Nos. 2,433,132 and 3,976,415 and French Pat. No. 2,527,976 wherein the separation of the mold elements at the part-line is measured and the result utilized to change the machine from the injection phase to a pulsing of the injection ram to maintain the part-line separation constant during the packing and curing of the mold. This system has not been found to be feasible for the manufacture of small, precision parts since it is nearly impossible to so precisely control the pulsing of the ram to pump the microscopic amounts of material necessary to achieve the desired volumetric accuracy of such parts. This process is also described in the article by Bernard Sanchagrin, entitled "Process Control of Injection Molding" in the mid-June, 1983 issue of Polymer Engineering and Science, Vol. 23, No. 8.
Other proposed solutions are exemplified by U.S. Pat. Nos. 2,671,247 and 3,859,400, which teach the control of the switch point of the injection molding machine by sensing the pressure within the mold itself. This system proposes sensing the pressure within the mold to shift the machine from the injection phase to the holding phase while the material cures within the mold. The measurement of the pressure within the mold does not satisfactorily reflect the multiple variables noted above. For example, if the material characteristics are held constant and the machine clamp force is allowed to vary, even if the pressure in the mold cavity remains constant, the part size produced will vary with the clamp force, increasing with reduced clamp force and vice versa. Similarly, with variations in material characteristics, if the viscosity of the material changes, as it may with changes in the injection temperature or with different batches of material, even if the amount of material injected into the mold is substantially constant, the varying viscosity will affect the resistance of the material to being forced into the mold cavity and thus the pressure transmitted into the mold cavity from the injection ram. Accordingly, it will be seen that measuring the pressure in the mold cavity will not truely reflect many of the variables that influence the process and the product.
Another proposed method of controlling injection molding processes is that taught by U.S. Pat. No. 3,940,465 wherein the measurement of the separation of the part-line is utilized to control the cure time of the injection molding cycle. This type of control fails to reflect all of the variables, noted above, which must be accomodated to accurately control part weight and dimension.
U.S. Pat. No. 4,135,873 teaches the measurement of the part-line separation and comparing the separation with a predetermined value and thereafter varying the injection pattern of the injection ram during the following molding cycle. This system does not provide control of the process on a real time basis, reflecting system conditions that are affecting the current cycle. Such a system merely reflects what occurred on the previous cycle, resulting in a tendency for the system to hunt rather than zero into a mode of operation which provides product consistency.
U.S. Pat. No. 4,131,596 teaches the measurement of the part-line separation to reduce the mold clamping pressure upon the measurement of a predetermined separation to minimize any damage to the mold due to flashing of the material at the part-line. This, of course, does not contribute to the control of product weight and dimension.
Japanese Patent Publication No. 11974 of 1978 discloses a method of controlling an injection molding machine wherein the part-line separation is measured and, upon reaching a predetermined reference separation, the machine is switched from a material filling mode to a dwelling mode. The mold separation is then measured and the maximum separation is determined. Thereafter, pressure during the dwell or curing phase of the mold cycle is controlled dependent upon the maximum separation reached to control the final mold separation value at the end of the cure time. Thereafter, the reference separation value for the switch point for the following cycle is changed to accommodate the variations in the machine operation detected during the first cycle. This system of control has the disadvantage that the switch point is determined by the preceeding cycle and thus does not reflect the conditions of the current cycle. This system of control thereafter attempts to adapt to the variations in the molding conditions existing during the current cycle by controlling the holding pressure during the cure phase of the cycle which can adversely affect part weight and density uniformity.
Each of the foregoing control systems has either been too complex and expensive and/or has not provided the requisite control related to all of the variables acting upon an injection molding process.
It has been found that the variables noted above are reflected in the molding process through the part-line opening which also reflects the consistency of the dimensions and weight of the molded product. During repeated molding cycles variations in the product from the molding process can range from under-filled mold cavities (short) to over-filled mold cavities (flashed). The aim point for the process will be somewhere between these two extremes to produce a product which meets the dimensional and weight tolerances established for that process and product. Further, it is known that mechanical and thermal strains are inherent in the molding process which are then transfered to the molded product, sometimes to the detriment of the dimensional stability and life of the product. The mechanical strains are produced by the clamping pressure necessary to hold the mold elements together against the force of the injection of the molten material. The thermal strains occur during the filling and packing of the mold with the high temperature molten material and its effect on the much cooler mold cavity walls, followed by the shrinkage of the material as it cures.
The present invention operates on the premise that a part-line and time window can be generated that directly correlates the weight and dimension of the part being molded for any given mold/machine/material combination, and that so long as the mold, machine, and material parameters remain constant, the part-line/time "signature" will remain relatively constant over a plurality of molding cycles. This part-line/time "signature" can then be used to generate a part-line and time window that can be employed to monitor and/or control the molding process.
Accordingly, the provision of a method for generating such a part-line/time signature which accurately represents the dimension, weight, and stress in a molded product would significantly enhance the productivity of the molding process.